The stromal microenvironment in which tumor cells reside influences many steps of carcinogenesis and cancer progression. The influences of the microenvironment are mediated, in large part, by paracrine signaling between epithelial cells and neighboring stromal fibroblasts. In this proposal, we will contribute to the elucidation of the complex stromal/epithelial interactions involved in the initiation and development of breast cancer by utilizing relatively novel in vitro and in vivo models to assess the effect of human fibroblasts and their cellular products, emphasizing insulin like growth factor II (IGF-II), on the growth of non-transformed and partially transformed ("pre-neoplastic") human breast epithelial cells. We will utilize the MCF10AT model of human proliferative breast disease, and the cell line from which the MCF10AT was originally derived, MCF10A cells. The hypothesis underlying this work is thus, there is a shift in insulin like growth factor (IGF) expression (from insulin like growth factor I (IGF-I) to IGF-II) in fibroblasts associated with carcinoma (FAC) and this shift is a key modulator of breast carcinogenesis. In support of this hypothesis, the proposed work will demonstrate the following: 1) When grown in three dimensional co-culture with non-transformed or partially transformed cells, FAC will result in an increase in the rate of cellular proliferation and a decreased rate of apoptosis of the co-cultured epithelial cells compared with epithelial cells grown with fibroblasts derived from benign breast (FAB). 2) The effect of increased proliferation and decreased apoptosis is, at least in part, a result of overexpression of IGF-II in FAC derived from breast carcinomas. 3) Overexpression of IGF-II in breast fibroblasts, when grown in vivo with MCF10AT cells will result in an increase in the number and degree of severity of proliferative lesions. An improved understanding of the role of fibroblasts and the IGF system in breast carcinogenesis is important to the development of effective preventive strategies.